There is a need for an accurate assessment of the dose delivery to the oncology patient during external beam radiation therapy. A direct assessment is virtually impossible due to the location of the radiation target inside the human body that is comprised of complex shapes and material compositions that have an affect on the radiation delivery, to say nothing of the complexity of the delivery controls that have evolved over the past decade. Great strides have been made in the planning and calculations of dose transport and deposition in the heterogeneous human anatomy; there are several independent types of calculation models that are used by these treatment planning systems (TPS), all with particular strengths and weaknesses. The clinician must ultimately rely on the TPS dose assessment tools to evaluate the treatment plan's potential for success and risk. One of these tools is the dose volume histogram (DVH), which is a statistical formulation of 3D dose coverage (of targets) and sparing (of critical structures) to allow a metric, or ranking, by which to assess, compare, and approve treatment plans.
There are a number of approaches to validate the planned dose delivery. The elemental approach is to validate all elements of the chain that lead from plan to delivery, starting with rigorous verification of the plan dose modeling (as described in Benedick Fraass, et al, American Association of Physicists in Medicine Radiation Therapy Committee Tasg Group 53: Quality assurance for clinical radiotherapy treatment planning“, Med. Phys., 25, (10), October 1998, pp 1773), eliminating all weakness of the TPS from the actual plan, performing rigorous quality assurance (QA) testing of the delivery system 2 (as addressed in G. J. Kutcher, et al, “Comprehensive QA for radiation oncology report of AAPM Radiation Therapy Committee Task Group 40”, Med. Phys. 21, 1994, pp 581), as well as the imaging system involved in the planning and which may be used during treatment with image guided radiotherapy (IGRT), and the fundamental processes of the mechanical setups and alignments that are required in the delivery room. The elements extend even further than this when considering the personnel, servicing, and software updates to the systems in planning and control. Such an elemental approach may be ideal but is also impractical in its administration and management of results.
Current approaches rely on validating sub-parts of the system with hopes of detecting sources of error that would affect the dose planning and delivery system as a whole. It is good to hone different QA tests to probe defined sub-systems. However, often these approaches are limited in that they may not guarantee that “good results” correlate with accurate dose to the patient nor do they project how “bad results” correlate with unacceptable dose to the patient. That is, the impact to the patient dose system is not quantified by the sub-system test. For example, a popular sub-system approach over the past eight years has been the dose map measurement on a QA phantom and the comparison of the measured to the planned dose map on that phantom. (see “MapCHECK” and “EPIDose” as described at www.sunnuclear.com and manufactured by Sun Nuclear Corp, Melbourne Fla.). This test method using MapCHECK™ or EPIDose™ provides a verification of the system's ability to deliver the patient's treatment plan on the QA phantom. First, the patient planning process is completed, which includes a treatment plan delivery from the radiation machine and a 3D dose distribution in the patient anatomy. Next, the TPS computes the dose in the QA phantom using the patient's treatment delivery plan. Then the measurement of the dose map in the QA phantom is made with the patient's treatment delivery plan and compared to the TPS computed dose in the QA phantom. Differences between the measured and planned map are errors caused by either delivery errors or dose computation errors. Such dose distribution errors may then be expected in the patient anatomy; the significance of the error to the treatment outcome is a judgment made by the clinician, aided by the comparison criteria, but generally without guidance of how the 3D dose to the patient or DVH changed due to the detected error.
One method does not involve a measurement; instead, the TPS plan for beam shaping is used in the calculation of the planned dose map on the homogeneous QA phantom, using a dose model algorithm that has been validated by a measuring device, such as the MapCALC™ product by Sun Nuclear (see “MapCALC” at www.sunnuclear.com and manufactured by Sun Nuclear Corp, Melbourne Fla.). The dose map comparison is made to the TPS dose map calculation on the QA phantom, very much like in the MapCHECK™ measurement comparison described above. This comparison method may be suitable as long as there is a comprehensive measurement
QA program on the machine delivery itself, and periodic comparison of the MapCALC™ dose map to the measurement dose map. If there are errors in the dose map comparison, the errors signify differences in the dose modeling between the TPS and MapCALC™. The clinician is not aware of the impact of the error on the DVH, i.e., is the error serious enough to investigate?
Other treatment plan QA validation methods seek to achieve QA by DVH estimation and subsequent comparison to the plan. They have been generally far more complicated because of their dose modeling in the heterogeneous structures of the patient. There are basically two categories, those that assume accurate delivery by capturing the planned beam fluence by various methods, (see Joseph O. Deasy, et al, “CERR: A computational environment for radiotherapy research,” Med. Phys. 30, (5), May 2003, 979) and those that measure, or nearly measure, the beam fluence (see U.S. Pat. No. 6,853,702 to Renner for “Radiation Therapy Dosimetry Quality Control Process”). Both calculate the dose to the patient from the beam fluence using a dose algorithm that is independent from TPS patient dose calculation. The dose modeling in the QA systems may be similar to or significantly different from the TPS dose modeling. After patient dose calculation, the QA system will then summarize the dose into a DVH analysis for the clinician. Without even considering the accuracy of the assumed or measured beam fluence, the fundamental problem is the confidence in dose modeling; there are now two dose models in competition, both computing a 3D dose distribution in the patient anatomy and summing the dose to create the DVH of interest. The clinician is faced with a decision as to which is correct if there is a significant difference. Furthermore, if the TPS has been rigorously commissioned prior to use, why would the clinician decide in favor of another less rigorous dose model? A TPS should be commissioned (via data entry, beam modeling, etc.) to be as accurate as it can possibly be, and once this is complete, consistency should be the rule, i.e. by using defined and validated processes, maintaining the performance, and quantifying and understanding its inherent limitations.